The polymerization of acrylic acid (AA) was performed under Co-60 irradiation in the presence of dibenzyl trithiocarbonate at room temperature, and well-defined poly(acrylic acid) (PAA) with a low polydispersity index was successfully prepared. The gel permeation chromatographic and H-1 NMR data showed that this polymerization displays living free-radical polymerization characteristics: a narrow molecular weight distribution (M-w/M-n = 1.07-1,22), controlled molecular weight, and constant chain-radical concentration during the polymerization. Using PAA-S-C(double bondS)-S-PAA as an initiator, the extension reaction of PAA with fresh AA was carried out under Co-60 irradiation, and the results indicated that this extension polymerization displayed controlled polymerization behavior.